1. Field of the Invention
The present invention generally relates to phase shifters in a distributed or coupled line technique. Such devices are used to phase shift radiofrequency signals with respect to each other, generally by 90°. Such devices can be found in association with balanced power amplifiers, mixers, power combiners/dividers, most often to shift the phase of radiofrequency signals corresponding to different paths.
2. Discussion of the Related Art
FIG. 1 is a block diagram of a circuit performing a 90° phase shift of two input signals. Such a circuit comprises two accesses IN0 and IN180, arbitrarily designated as inputs, intended to receive two differential signals Pin 0° and Pin 180° (in phase opposition), and two accesses OUT0 and OUT90, arbitrarily designated as outputs, intended to provide signals Pout 0° and Pout 90° in phase quadrature. Such a device most often is bi-directional, that is, it may be used, according to its assembly in an electronic circuit, to shift the phase of differential input signals to obtain signals in phase quadrature or to increase the phase shift of a signal in quadrature to obtain a signal in phase opposition.
FIG. 2 is a schematic block diagram of a conventional example of a radiofrequency transmit circuit using a phase shifter (block 1). This phase shifter belongs to a radiofrequency transmission head 10 and is interposed between differential inputs RFin+ and RFin− receiving a signal to be transmitted and two amplifiers 11 and 12 (PA) intended to receive radiofrequency signals in phase quadrature. Signals RFin+ and RFin− are in phase opposition, especially for better noise immunity. Circuit 10 is powered by a generally D.C. voltage Valim. The respective outputs of amplifiers 11 and 12 provide amplified signals ARF0 and ARF90 to a combiner 15, possibly after crossing of impedance matching circuits 13 and 14 (MATCH) (shown in dotted lines). The function of combiner 15 is to add two input powers in a common path to form an RF signal sent onto an antenna 16 for the transmission. A coupler may be added to combiner 15 to extract data proportional to transmitted power Pout to adjust the gains of amplifiers 11 and 12.
A similar architecture may be used for a reception chain. In this case, the combined RF access is used as an input terminal while two distributed accesses provided by the combiner in phase quadrature are sent onto two reception inputs of a radiofrequency reception head comprising a phase shifter for providing signals in phase opposition for the processing circuits.
The forming of a phase shifter may use so-called lumped-element techniques (association in an electric diagram of lumped inductive and capacitive elements) or with distributed or coupled lines (conductive lines arranged sufficiently close to each other to generate an electromagnetic coupling).
Phase shifters formed of lumped elements are limited to very narrow frequency bands (narrower than some ten MHz).
The present invention more specifically applies to distributed phase shifters which may process wider frequency bands.
FIG. 3 shows a conventional example of a phase shifter 1′ made in integrated form with distributed lines. In FIG. 3, the details of the lines have not been shown and only functional blocks are illustrated. In such an embodiment, the phase shifter uses both a balun transformer 17 and a combiner 18 (COMB). Transformer 17 is in charge of transforming the differential signals present on input terminals IN0 and IN180 into an intermediary common-mode signal INT. Signal INT is divided by combiner 18 to form two signals in phase quadrature provided on outputs OUT0 and OUT90.
A disadvantage of phase shifter 1′ of FIG. 3 is its large size. Indeed, in a technique with distributed lines, the line lengths are by a quarter or an eighth of the wavelength of the central frequency of the frequency band to be processed, and a multiplication of the devices increases the size.
Another disadvantage of phase shifter 1′ of FIG. 3 is that it exhibits high insertion losses.